This invention relates to optoelectronic devices, and more particularly to a method of achieving nearly arbitrary two-dimensional spatial gain profiles within an optoelectronic device and its specific application to a semiconductor laser, to provide a new type of laser, the tailored gain broad area semiconductor laser, which is capable of high power operation with very narrow, single lobed farfield patterns.
Conventional semiconductor lasers are capable of emiting several tens of milliwatts of optical power into a single beam ten to twenty degrees wide in the junction plane (the lasers described herein have been shown capable of emitting 200 milliwatts of optical power into a single beam only 21/2.degree. wide). Considerable effort has gone into finding methods of increasing the power output and decreasing the beamwidth of a semiconductor laser. In principle, one method by which this might be done is to increase the width of the laser in the lateral direction to make a "broad area" laser. (The term "broad area" laser usually refers to a semiconductor diode laser with a width in the lateral direction of greater than about 10 to 15 .mu.m). Conventional broad area semiconductor lasers have a nearly uniform lateral spatial gain profile. As a direct result of this uniform gain profile, such devices have very wide, poorly characterized, and unstable farfield patterns. These undesirable farfield patterns result from two physical effects.
First, the presence of a nonlinear interaction between the carriers and the optical field in a conventional semiconductor laser with a uniform spatial gain profile produces filaments, so-called because a photomicrograph of an operating device exhibits small areas of enhanced optical intensity with a filamentary structure. This interaction effectively forms a small waveguide 3 to 12 .mu.m wide within the larger waveguide defined by the entire broad area laser. As a result of translational invariance within a conventional uniform gain broad area laser, the filaments become unstable and move about randomly. The complicated motions and interactions of the many filaments in a conventional uniform gain broad area laser are one cause of the poor beam quality characteristic of these devices. Therefore, if a laser's power output is to be increased by increasing the laser's width, some method of stabilizing the filaments must be found. In conventional semiconductor lasers, this is usually achieved by making the laser's width narrow enough so that only one filament can form.
A second problem which must be overcome comes about because the waveguide in a broad area laser can support many optical modes. In a conventional uniform gain broad area laser, only the fundamental mode will have a predominately single-lobed farfield pattern. Thus, if the laser's farfield pattern is to be single-lobed and diffraction limited (i.e., as narrow as possible), the fundamental mode must be the only lasing mode. All other modes must be discriminated against. This is not possible in a conventional broad area laser with a uniform spatial gain profile because many of the undesirable higher order modes have modal gains very close to that of the fundamental, and will therefore lase along with the fundamental. Without some means of tailoring the spatial gain profile so as to flavor the fundamental, the only method of achieving single lobed farfield operation is to make the laser narrow enough so that the waveguide supports only the fundamental mode, making it the sole lasing mode.
Thus, the twin problems of filamentation and lateral mode control may be solved by the simple expedient of limiting the width of the laser, typically 5 to 10 .mu.m. However, limiting the width of the laser stripe also limits the laser's maximum power output and minimum beamwidth. New semiconductor laser designs which achieve high power operation by increasing the laser's width must solve both the filamentation and lateral mode control problems. This cannot be done without creating a nonuniform spatial gain distribution within the laser--i.e., by employing some form of gain tailoring.
When compared with the flexibility inherent in the halftone process for achieving two-dimensional gain tailoring, the prior art of gain tailoring is very primitive, depending critically on material parameters which cannot be widely varied, or in laser array structures offering at best only crude one-dimensional control (C. P. Lindsey, E. Kapon, J. Katz, S. Margalit, and A. Yariv, "Single Contact Tailored Gain Phased Array of Semiconductor Lasers", Appl. Phys. Lett" 45(7), 1984, pp. 722-724).